Heretofore, a liquid such as a coating material including solid particles has been handled and dispensed from a dispensing valve by the following three methods because the solid particles easily precipitate. Note that, the expression “dispensing a liquid” as used herein comprehends both dispensing (dispensing the liquid as it is) and spraying (spraying the liquid, that is, atomizing it and then dispensing it).
(1) A method in which a liquid stirred by a large-sized apparatus in a storage tank is divided and stored in syringes or small vessels and used right away.
(2) A method as proposed by JP 63-119877 A, in which a liquid in one of two pressure vessels is pressurized with compressed air, the air of the other vessel is opened to move the liquid through a liquid flow passage between the two vessels, and an auto dispensing valve as a dispensing valve is provided at an intermediate portion of the flow passage to dispense the liquid while the liquid is moving. This operation is carried out alternately between the vessels to prevent the precipitation of the solid particles.
(3) A method in which a circulation circuit is formed from, for example, a pump dispensing port to an auto dispensing valve and a pump suction port by using a pump or the like to circulate a liquid forcedly to a portion near the needle and valve seat of the auto dispensing valve. For example, a dispersion (dispersion type liquid including solid particles) of a mixture of carbon particles and a binder solution which is spray coated on the inner surface of an alkali dry battery to improve its performance is circulated at a relatively high liquid pressure in order to re-disperse secondary agglomerates of the particles. Since stable coating can be performed by employing this method while preventing the precipitation of the carbon particles, it is globally used.
However, in the method mentioned in the above item (1), in the case of a liquid having a low viscosity in the range of 3,000 mPa·s or less, particularly about 1 to 500 mPa·s, the precipitation of the solid particles, although depending on the specific gravity and size of the particles, is so fast that there is a big difference between the quality of the liquid at the start of dispensation and the quality of the liquid during dispensation or at the end of dispensation, and particularly the content of the particles is the major concern. Further, the precipitated particles accumulate on a portion near the valve and the valve seat of the auto-dispensing valve, often causing a dispensation failure.
In the method mentioned in the above item (2), the flow rate of the liquid is determined by the level of air pressure. Therefore, control of a period of time before the subsequent step, that is, from the time when the liquid moves from the first tank to the second tank to the time when the liquid moves from the second tank to the first tank is affected only by the pressure of compressed air. Therefore, when a commercially available air regulator is used, a low-viscosity liquid filled in a syringe having a small capacity of about 5×10−6 m3 to 30×10−6 m3 (5 cc to 30 cc) for instance, is moved to a syringe on the opposite side instantaneously, in less than 1 second when pressurized at a pressure of 0.05 MPa which is the minimum graduation. Thereby, problems arise that the operation of dispensing the liquid by the dispensing valve cannot be continued for a desired period of time and dispensation cannot be carried out stably. The method also involves problems such as the inclusion of air and the difficulty of dispensing a predetermined amount of the liquid stably.
Further, even if an air regulator equipped with a gauge having a minimum graduation of 0.001 MPa is used to apply pressure to the liquid, the moving time of the liquid in the syringe having a capacity of 30×10−6 m3 (30 cc) is in the order of second and the moving direction must be changed frequently to carry out an automatic operation. Also, the frequent interruption of work cannot be avoided even when a large vessel having a capacity of several liters is used.
Thus, to prevent the interruption of work at the time of changing of the moving direction, as proposed in JP 60-5251 A, there is a method in which three coating material tanks are used for the stable supply of a powder slurry coating material. In this method, pressurized air is supplied to the first tank to always maintain a fixed pressure, and the powder slurry coating material is pumped to the third tank through a coating gun at the same liquid pressure as the pressure of the pressurized air. When the level of the first tank lowers, pressurized air is supplied to the second tank to pump the coating material through the second tank and dispense it from the coating gun. In this method, while pumping from the second tank is being stabilized, 10 seconds of simultaneous pumping from the first and second tanks is required.
In general, these tanks have a capacity of 18×10−3 m3 to 30×10−3 m3 (18 liters to 30 liters). Therefore, this method is not suitable for the above-mentioned syringes, which are small vessels.
Further, the above-mentioned two methods disclosed by JP 63-119877 A and JP 60-5251 A involve a problem that a coating film adhered to the wall of a tank is dried upon its contact with a dry gas as the level of the coating material lowers because a pressure source is a gas such as compressed air. Since the powder slurry and the dispersion contain a solution of a polymer such as a binder in addition to inorganic or organic solid particles, after they are dried, the polymer solution component which has not been re-dissolved is no better than a foreign matter.
Furthermore, it is known in the industry that when compressed gas such as compressed air comes into contact with a low-viscosity liquid rich with a solvent in particular, a part of the gas dissolves in the liquid. Therefore, a quality problem often occurs because micro-bubbles are contained in the dispensed liquid.
In the method mentioned in the above item (3), a special plunger pump which is free from pulses and the accumulation or agglomeration of particles in the circuit and which is not worn down by solid particles must be used. This apparatus is large in size and expensive and also requires one (1) gallon (about 3.8×10−3 m3 (3.8 liters)) or more of a coating material for stable circulation. Therefore, it is not suitable as a tester for testing with several 10×10−6 m3 (several tens of cc) of a coating material which is required for the laboratory-level development of a material, and a huge amount of money has been spent on the development of a material. In addition, a large amount of a solvent has been required for the cleaning of the inside of the circuit at the end of work and most of the coating material in the circuit cannot be used because it contains a cleaning solvent.
In the past several years, the number of expensive materials has been growing due to progress in the development of functional coating materials. Such materials include a dispersion containing inorganic particles having a uniform particle size distribution and a size of several micrometers or less, or of a nanometric level in some cases, a powder slurry containing polymer particles uniform in particle size, an electrode-ink for the electrodes of fuel cells as proposed in U.S. Pat. No. 5,415,888 B and the like, and an electrode-ink having super fine particles of platinum in a nanometric order carried on a carbon nanotube. Some of those coating materials not uncommmonly cost several million yen per kilogram, and an apparatus and method, which not only allow for high-quality coating but also are capable of making the most of a minimum amount of a coating material, are desired.